Printing apparatus and printing method

ABSTRACT

A smooth, uniform image is produced by minimizing the occurrence of satellites of secondary color and dispersing landing positions of the satellites as uniformly as possible. For this purpose, the printing operation performed so that satellites of the two inks (cyan and magenta ink, foe example) ejected toward the same pixel are separated and landed on opposite sides of the main dots on the same pixel. This makes the distribution of satellites uniform and makes individual satellites less noticeable, maintaining the uniformity of an image.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/JP2006/313592, filed on Jul. 7, 2006. The entire disclosure of thisprior application is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink jet printing apparatus andmethod to form a uniform image.

2. Description of the Related Art

A printing apparatus of an ink jet printing system (hereinafter referredto as an ink jet printing apparatus) performs a printing operation byejecting ink from a print head onto a print medium and can easily beupgraded to a higher resolution, compared with other printing systems.The ink jet printing apparatus also has advantages of high speedprinting capability, low noise and low cost. As there are growing needsfor color output in recent years, a printing apparatus capable ofproducing high-quality printed images matching silver salt pictures inquality has been developed.

The ink jet printing apparatus incorporates a print head having aplurality of print elements (electrothermal transducer or piezoelectricelement) densely arrayed therein for higher printing speed. Also for acolor printing capability, many printing apparatus are provided with aplurality of such print heads.

FIG. 1 shows a construction of main components of a general ink jetprinting apparatus. In the figure, denoted 1101 are ink jet cartridges.Each of these has a combination of an ink tank containing one of fourcolors, black, cyan, magenta and yellow, and a print head 1102corresponding to the ink.

FIG. 2 shows a group of the ejection openings for one color arrayedcorresponding to the print elements of the print head 1102, as seen froma direction of arrow Z of FIG. 1. In the figure, denoted 1201 areejecting openings that number d and are arranged at a density of Dopenings per inch (D dpi). Hereinafter, a constitution including a printelement and an opening corresponding to that is referred to as a nozzle.

Referring again to FIG. 1, reference number 1103 represents a paper feedroller, which, together with an auxiliary roller 1104, holds a printmedium P and rotates in the direction of arrow to feed the print mediumP in the direction of arrow Y (subscan direction). Denoted 1105 are apair of supply rollers that supply the print medium P. The paired supplyrollers 1105, as with the rollers 1103 and 1104, hold the print medium Pbetween them and rotate at a slightly lower speed than the paper feedroller 1103, thereby applying an adequate level of tension to the printmedium.

Denoted 1106 is a carriage that supports the four ink jet cartridges1101 and moves them as the cartridges perform a scan. The carriage 1106stands by at a home position h shown with a dashed line when theprinting operation is not performed or when a recovery operation on theprint head 1102 is executed.

When a print start command is entered into the printing apparatus, thecarriage 1106 standing by at the home position h moves in the Xdirection (main scan direction) and at the same time the print heads1102 on the carriage eject inks at a predetermined frequency from thenozzles 1201, forming a band of image d/D inch wide on the print medium.After the first printing scan is finished and before the second printingscan starts, the paper feed roller 1103 rotates in the direction ofarrow to feed the print medium a predetermined distance in the Ydirection. These main printing scan and feeding operation are alternatedrepetitively to produce an image in a stepwise fashion.

Such an ink jet printing apparatus often employs a multi-pass printingmethod. The multi-pass printing method will be briefly explained below.

In the multi-pass printing, image data that can be printed in one mainprinting scan is thinned by a mask pattern before executing the mainprinting scan. Further, in the next printing scan, image data that isthinned by a mask pattern complementary to the already used mask patternis printed. Between each printing scan, a feed operation is performed tofeed the print medium a distance shorter than the print width of thehead.

In the case of a 2-pass printing, for example, a mask pattern used ineach main printing scan thins the image data by about 50%. The distancethat the print medium is fed by the feed operation is one-half the printwidth. By repeating the above printing operation, dots arrayed on a lineleading to the main scan direction are printed by two different nozzles.Thus, since the print data is divided into halves and distributed amongthe two different nozzles, even if individual nozzles have some ejectingvariations, an image produced is smoother than that produced by a 1-passprinting that does not use the multi-pass printing. Although the 2-passprinting has been explained here, the image produced by the multi-passprinting can be made smoother by increasing the number of passes(division number). This, however, results in an increased number of mainprinting scans and feed operations and therefore an increased outputtime. To reduce the output time as much as possible, a bidirectionalmulti-pass printing has become a mainstream in recent years which ejectsink in both forward and backward directions.

When ink is ejected from the nozzles of the ink jet print head, fine subdroplets of ink may be ejected along with main droplets that areintended to form an image. In the following description, dots formed bythe main droplets are called main dots and dots formed by sub dropletssatellites. The above relation between the main droplet and the subdroplet holds in one ejection. The one ejection referred to here is anejection performed in response to one electric signal. The sub dropletis characterized by a slower ejection speed and a smaller volume thanthose of the main droplet. It is noted, however, that the satellites arenot always smaller in size than the main dots.

FIGS. 3A to 3D show landing positions on a print medium of a main dotand a satellite. In these figures, 1301 represents a main dot and 1302 asatellite. An arrow shown in an upper part of these figures indicates adirection in which a carriage moves during the ejection operation. Anarrow shown in a lower part of the figures indicates a direction inwhich a droplet is ejected.

FIG. 3A shows dots formed when the direction of ejection is vertical tothe print medium. Normally if the print head is not inclined, theejection face of the print head is parallel to the print medium and thedirection of ejection is therefore vertical. Generally the sub dropletis slower in ejection speed than the main droplet and therefore lands onthe print medium lagging behind the main droplet. During ejection, thecarriage is moving in the direction of arrow 1303 in the figure, so thecarriage speed is added to the ejection speed of the droplet, with theresult that the landing time difference results in a landing positiondifference in the main scan direction.

FIG. 3B illustrates dots formed when the direction of ejection includesa component of the carriage movement. If the ink droplet ejectiondirection has some inclination due to various factors, such as a nozzlematerial swelling or the ink to be ejected being pulled into the liquidchamber, the ejection face of the head is not parallel to the printmedium, forming dots as shown in FIG. 3B. In that case, the velocitycomponents of the main droplet and sub droplet are each given thecomponent of arrow 1304. Thus, the distance between the main dot 1301and the satellite 1302 in the main scan direction further increases.

FIG. 3C illustrates dots formed when the ejection direction has aninclination opposite to that of FIG. 3B and includes a component (arrow1305) opposite to the direction of carriage movement. In this case, thevelocity components of the main droplet and sub droplet are the ejectiondirection component 1305 subtracted from the carriage velocity component1303. Thus, the distance between the main dot 1301 and the satellite1302 is shorter than that of FIG. 3A. FIG. 3C shows the satellitecontained in the main dot when they land.

FIG. 3D illustrates dots formed when the velocity component is the sameas that of FIG. 3C but the volume of a sub droplet is smaller. Subdroplets tend to have a smaller ejection speed as their volumedecreases. Thus, the smaller the sub droplet, the larger the landingtime difference between the sub droplet and the main droplet andtherefore their distance. FIG. 3D shows a satellite formed separate fromthe main dot because of a larger landing time difference between themain droplet and the sub droplet than that of FIG. 3C.

As described above, the print position of satellite varies depending onvarious factors. When a bidirectional multi-pass printing is performed,dots formed in the forward scan and dots formed in the backward scan mixin the same image area (for example, the same pixel, the same pixel lineor the same pixel area having M×N pixel).

FIG. 4 shows a variety of dot landing states when a bidirectionalmulti-pass printing is performed on a 2×2-pixel area. It is seen thatthe printed positions of satellites are inverted relative to the maindots depending on whether individual pixels are printed in the forwardor backward main scan. In FIG. 4, a right-pointing arrow denotes aforward direction, a large circle with diagonal lines denotes a main dotprinted by the carriage scanning in the forward direction, and a smallcircle with diagonal lines denotes a satellite printed by the carriagescanning in the forward direction. Furthermore a left-pointing arrowdenotes a backward direction, a large white circle denotes a main dotprinted by the carriage scanning in the backward direction, and a smallwhite circle denotes a satellite printed by the carriage scanning in thebackward direction.

As long as the satellites described above, if produced, are printed atthe same position as the main dots or small enough compared with themain dots, no problem occurs in image quality. However, with a printhead developed in recent years to eject very small ink droplets withhigh resolution, the main dots themselves have much smaller diametersand therefore the presence of satellites cannot be ignored.Particularly, when a secondary color is produced by overlapping twodifferent inks, the problem becomes more serious.

FIGS. 5A to 5C show a case where cyan dots and magenta dots areoverlapped to produce a blue color. As shown in the figure, two bluedots are formed in a 2×2-pixel area by moving the carriage in thedirection of arrow. Here it is assumed that two print heads for cyan andmagenta have the same satellite producing conditions. A satellitecomposed of two overlapping color dots is formed by the side of eachblue dot formed of two main droplets. The satellites, formed byoverlapping two different colors, are more conspicuous than when theyare formed of a primary color, having greater effects on an image. Ifsuch distinctive satellites are produced unevenly, the uniformity isimpaired, deteriorating the image quality.

To deal with the unevenness in landing position of satellites, somemeasures have already been proposed. For example, Japanese PatentApplication Laid-open No. 2003-053962 discloses a technology thatcontrols the feed distance of a print medium such that it includes atleast an odd and even number of times the value of 1/D (D=printingresolution in the sub scan direction), in order to disperse the landingpositions of satellites as possible and produce a uniform image.

With the method disclosed in the Japanese Patent Application Laid-openNo. 2003-053962, however, a pixel in which satellites land on both sidesof a main dots and a pixel in which satellites land insides of a maindots are arranged alternately. It is insufficiency for uniformity ofimage. Further, the method disclosed in the application provides arestriction on the control of transport distance of the print medium.Moreover, this technology does not take the secondary color describedabove into consideration, leaving the problem of easily noticeablesecondary color satellites unsolved.

SUMMARY OF THE INVENTION

The present invention has been accomplished to solve the above-mentionedproblems and it is an object of this invention to provide an ink jetprinting method and an ink jet printing apparatus which can producesmooth, uniform images by minimizing the forming of satellites ofsecondary color as practically as possible and dispersing the landingpositions of satellites as uniformly as possible.

The first aspect of the present invention is an ink jet printingapparatus for printing an image on a print medium by using a print headwhich can eject at least a first ink and a second ink, the second inkbeing different from the first ink at least in color or ejecting volume,the ink jet printing apparatus comprising: means for main-scanning theprint head relative to the print medium in a forward direction and in abackward direction; and means for executing ejections of the first inkand the second ink toward a same pixel on the print medium in main scansof different directions; wherein a satellite of the first ink ejectedtoward the same pixel lands shifted in the forward or backward directionwith respect to main dots of the first and second ink that land on thesame pixel and a satellite of the second ink lands shifted, with respectto the main dots of the first and second ink, in a direction oppositethe direction in which the satellite of the first ink shifts.

The second aspect of the present invention is an ink jet printingapparatus for printing an image on a print medium by using a print headhaving at least a first opening to eject a first ink and a secondopening to eject a second ink, the second ink being different from thefirst ink at least in color or ejecting volume, the ink jet printingapparatus comprising: means for main-scanning the print head relative tothe print medium in a forward direction and in a backward direction; andmeans for executing ejections of the first ink and the second ink towardthe same pixel on the print medium in main scans of differentdirections; wherein a plurality of pixels toward that both the first andsecond ink are ejected comprise a first pixel toward that the first inkis ejected in the main scan of the forward direction and the second inkis ejected in the main scan of the backward direction and a second pixeltoward that the first ink is ejected in the main scan of the backwarddirection and second ink is ejected in the main scan of the forwarddirection; wherein a satellite of the first ink lands shifted in theforward direction and a satellite of the second ink lands shifted in thebackward direction, with respect to landing positions of main dots ofthe first and second ink printed on the first pixel; wherein a satelliteof the first ink lands shifted in the backward direction and a satelliteof the second ink lands shifted in the forward direction, with respectto landing positions of main dots of the first and second ink printed onthe second pixel.

The third aspect of the present invention is an ink jet printingapparatus for printing an image on a print medium by using a print headwhich can eject at least a first ink and a second ink, the second inkbeing different from the first ink at least in color or ejecting volume,the ink jet printing apparatus comprising: means for main-scanning theprint head relative to the print medium in a forward direction and in abackward direction; and means for executing, in main scans of differentdirections, ejections of the first ink and the second ink forward ontopixels adjoining in a direction perpendicular to the direction of mainscans on the print medium; wherein a satellite of the first ink ejectedtoward the one of the adjoining pixels lands shifted in the forward orbackward direction with respect to main dots of the first ink landed onthe one pixel and a satellite of the second ink ejected toward the otherof the adjoining pixels lands shifted, with respect to the main dots ofthe second ink landed on the other pixel, in a direction opposite thedirection in which the satellite of the first ink shifts.

The fourth aspect of the present invention is an ink jet printingapparatus for printing an image on a print medium by using a print headhaving at least a first opening to eject a first ink and a secondopening to eject a second ink, the second ink being different from thefirst ink at least in color or ejecting volume, the ink jet printingapparatus comprising: means for main-scanning the print head relative tothe print medium in a forward direction and in a backward direction; andmeans for executing, in main scans of different directions, ejections ofthe first ink and the second ink onto pixels adjoining in a directionperpendicular to the direction of main scans on the print medium;wherein the adjoining pixels toward that the first and second ink areejected comprise a first pixel toward that the first ink is ejected inthe main scan of the forward direction and a second pixel toward thatthe second ink is ejected in the main scan of the backward direction;wherein a satellite of the first ink lands shifted in the forwarddirection, with respect to a landing position of a main dot of the firstink ejected toward the first pixel and satellite of the second ink landsshifted in the backward direction, with respect to a landing position ofa main dot of the second ink ejected toward the second pixel.

The fifth aspect of the present invention is an ink jet printing methodfor printing an image on a print medium by using a print head which caneject at least a first ink and a second ink, the second ink beingdifferent from the first ink at least in color or ejecting volume, theink jet printing method comprising the steps of: main-scanning the printhead relative to the print medium in a forward direction and in abackward direction; and executing ejections of the first ink and thesecond ink onto the same pixel on the print medium in main scans ofdifferent directions; wherein a satellite of the first ink ejectedtoward the same pixel lands shifted in the forward or backward directionwith respect to main dots of the first and second ink that land on thesame pixel and a satellite of the second ink lands shifted, with respectto the main dots of the first and second ink, in a direction oppositethe direction in which the satellite of the first ink shifts.

The sixth aspect of the present invention is an ink jet printing methodto print an image on a print medium by using a print head which caneject at least a first ink and a second ink, the second ink beingdifferent from the first ink at least in color or ejecting volume, theink jet printing method comprising the steps of: main-scanning the printhead relative to the print medium in a forward direction and in abackward direction; and executing, in main scans of differentdirections, ejections of the first ink and the second ink toward pixelsadjoining in a direction perpendicular to the direction of main scan onthe print medium; wherein a satellite of the first ink ejected towardone of the adjoining pixels lands shifted in the forward or backwarddirection with respect to main dots of the first ink landed on the onepixel and a satellite of the second ink ejected toward the other of theadjoining pixels lands shifted, with respect to the main dots of thesecond ink landed on the other pixel, in a direction opposite thedirection in which the satellite of the first ink shifts.

According to above construction, since landing positions of satellitesare dispersed uniformly, images of higher level of uniformity areprovided.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a construction of main components of an ink jetprinting apparatus applicable to the present invention;

FIG. 2 is a schematic diagram showing nozzles for one color arranged ina print head;

FIGS. 3A to 3D are explanatory views showing landing positions on aprint medium of a main dot and a satellite;

FIG. 4 illustrates various landing states when a bidirectionalmulti-pass printing is performed on a 2×2-pixel area;

FIGS. 5A to 5C illustrate dot positions when a blue is produced byoverlapping cyan and magenta dots;

FIG. 6 is a block diagram showing a control configuration of the ink jetprinting apparatus according to one embodiment of this invention;

FIG. 7 is a schematic diagram showing arrangements of nozzles in theprint head applied to the embodiment of this invention;

FIGS. 8A and 8B are schematic diagrams showing characteristics of maskpatterns applied to the embodiment of this invention;

FIGS. 9A to 9C show dot landing states when a blue, a secondary color,is produced by applying the masks of the first embodiment;

FIG. 10 illustrates examples of fixed mask patterns of 4×4 pixels;

FIGS. 11A to 11C illustrate how a 4-pass bidirectional multi-passprinting is performed by using fixed mask patterns;

FIG. 12 show dot landing states when image data is printed using randommask patterns;

FIG. 13 illustrates dot arrangements of an image completed by four mainprinting scans;

FIGS. 14A and 14B illustrate images completed in a wider area (16×16pixels) using a fixed mask and a random mask;

FIG. 15 is a schematic diagram showing arrangements of nozzles in theprint head applied to the embodiment of this invention;

FIG. 16 is a schematic diagram showing mask patterns applied to theembodiment of this invention;

FIGS. 17A and 17B illustrate images in a wider area (8×8 pixels) when ablue, a secondary color, is printed by applying a conventional mask anda mask of the first embodiment;

FIG. 18 is a schematic diagram showing arrangements of nozzles in aprint head applied to a third embodiment of this invention;

FIG. 19 is a schematic diagram showing mask patterns applied to thethird embodiment;

FIGS. 20A and 20B illustrate dot landing states when large dots andsmall dots are printed on pixels that adjoin each other in the nozzlearrangement direction, by applying the mask of the third embodiment;

FIGS. 21A and 21B illustrate dot landing states when large dots andsmall dots are printed on pixels that adjoin each other in the nozzlearrangement direction, by applying a conventional mask;

FIG. 22 is a schematic diagram showing mask patterns applied to a fourthembodiment of this invention;

FIG. 23 is a diagram showing directions in which dots are printedthrough a mask pattern A in the fourth embodiment;

FIGS. 24A and 24B illustrate dot landing states when a blue is producedby using large dots and small dots and the mask of the fourthembodiment;

FIGS. 25A and 25B illustrate dot landing states when a blue is producedby using large dots and small dots and a conventional mask; and

FIG. 26 is a schematic diagram showing examples of random mask patternsapplicable to the embodiment.

DESCRIPTION OF THE EMBODIMENT

Now, by referring to the accompanying drawings, embodiments of thisinvention will be described in detail.

First Embodiment

This embodiment applies the ink jet printing apparatus described in FIG.1.

FIG. 6 is a block diagram showing a control configuration of the ink jetprinting apparatus of this embodiment. In the figure, a CPU 700 controlsvarious parts described later and executes data processing. The CPU 700performs, through a main bus line 705, a head drive control, a carriagedrive control and data processing according to programs stored in a ROM702. The ROM 702 stores a plurality of mask patterns used in a printingoperation characteristic of this embodiment, as well as programs. A RAM701 is used as a work area for data processing by the CPU 700. The CPU700 also has memories such as hard disks, in addition to the ROM 702 andRAM 701.

An image input unit 703 has an interface with a host device not shownwhich is connected exteriorly, and temporarily holds an image datasupplied from the host device. An image signal processing unit 704executes data processing, such as color conversion processing andbinarization processing.

An operation unit 706 has keys for an operator to enter control inputs.

A recovery system control circuit 707 controls a recovery operationaccording to a recovery processing program stored in the RAM 701. Thatis, the recovery system control circuit 707 drives a recovery systemmotor 708 to operate a cleaning blade 709, a cap 710 and a suction pump711 for the print head 1102.

A head drive control circuit 715 controls the operation of print element(electrothermal transducers in this embodiment) installed in individualnozzles of the print head 1102 to cause the print head 1102 to execute apreliminary ejection and a printing ejection. Further, a carriage drivecontrol circuit 716 and a paper feed control circuit 717 also controlthe movement of the carriage and the feeding of paper according toprograms.

A substrate of the print head 1102 in which electrothermal transducersare installed is provided with a heater, which heats the ink in theprint head to a desired set temperature. A thermistor 712 is similarlyprovided in the substrate and measures essentially a temperature of theink in the print head. The thermistor 712 may be installed outside thesubstrate as long as it is located near the print head.

FIG. 7 shows an arrangement of ejecting openings (an arrangement ofnozzles) in the print head 1102 applied to this embodiment. In thefigure, denoted 801 is a nozzle column for a black ink, 802 a nozzlecolumn for a cyan ink, 803 a nozzle column for a magenta ink, and 804 anozzle column for a yellow ink. These four color ink nozzles eachcomprise an even nozzle column and an odd nozzle column. In the blackink, for example, 801 a represents the odd nozzle column and 801 brepresents the even nozzle column. By taking the nozzle column 801 forexample, the arrangement of nozzles will be explained in detail.

The odd nozzle column 801 a and the even nozzle column 801 b each have128 nozzles arrayed at 600 dpi, with the odd and even nozzle columns 801a, 801 b staggered in a Y direction (sub scan direction) by 1200 dpi.That is, ejecting ink from the print head as it scans in an X direction(main scan direction) can print a strip of image, about 5.42 mm wide, ata resolution of 1200 dpi in the sub scan direction.

Nozzle columns of other colors also have the similar construction tothat of the black nozzle column 801. These four color nozzle columns arearranged side by side in the main scan direction, as shown.

Next, a multi-pass printing method used in the printing apparatus ofthis embodiment will be explained.

FIG. 26 is a schematic diagram showing examples of random mask patternsapplicable to this embodiment. In the figure, individual squaresrepresent a pixel, a minimum unit area where a dot is to be printed ornot to be printed. Black squares represent pixels that permit theprinting of an ink dot during the associated printing scan (printpermission pixel) and blank squares represent pixels that do not permitthe printing of an ink dot during the associated printing scan (printnon-permission pixel). A random mask pattern is a mask pattern in whichprint permission pixels are arranged randomly and non-periodically. Anon-periodic mask pattern like this has the characteristics of notsynchronizing an image data which has periodicity. Although a maskpattern having a size of 16×16 pixel is used for example, it ispreferred that the size in main scan direction is larger. In thisembodiment, a mask pattern having a size of 1028 pixel in main scandirection is applied, which is not shown in figure. A random maskpattern can be made by using a method disclosed in Japanese PatentApplication No. 3176181.

Four mask patterns for four-pass printing shown in the figure arecomplementary to one another. In each printing scan, the CPU 700 takes alogical AND of one of mask patterns A-D stored in the ROM 702 and theprint data to be print by each nozzle column, thus generating dataaccording to which ink is to be ejected in the associated printing scan.

FIGS. 8A and 8B are schematic diagrams showing how the mask patterns A-Dare used. Shown here are mask patterns that correspond to the cyannozzle column 802 and the magenta nozzle column 803 used in a 4-passbidirectional multi-pass printing. The odd and even nozzle columns, eachcomposed of 128 nozzles, have their nozzles divided into eight blocks of16 nozzles in the direction of sub scan direction. Each of the blocks isassigned with one of the four mask patterns A-D. In the figure, fourprinting scans, first to fourth scan, are shown and, between eachprinting scan, a paper feed operation is done to feed the print medium adistance corresponding to two blocks. Here, the print head is shown tomove relative to the print medium.

Reference symbols A-D of FIGS. 8A and 8B correspond to blocks in nozzlecolumns that apply the mask patterns A-D shown in FIG. 26. Theyrepresent four different patterns that are exclusive and complementaryto one another. That is, an image to be printed in one and the sameimage area of a print medium is completed by successively applying oneof the four different mask patterns A-D to each of the four mainprinting scans.

FIG. 8B shows a conventional, commonly used mask pattern arrangement. Itis conventionally a common practice to use the same kind of mask patternin all nozzle columns in the same printing scan, whether the columns areeven nozzle columns, odd nozzle columns or different color nozzlecolumns. That is, in the example shown, during the first scan all nozzlecolumns use the mask pattern A; during the second scan they use the maskpattern B; during the third scan they use the mask pattern C; and duringthe fourth scan they use the mask pattern D. In the fifth and subsequentscan, the mask patterns are again used in the same order beginning withA and the main printing scans are repeated with this order of maskpatterns maintained.

If a blue, a secondary color, is to be produced using such maskpatterns, pixels that were printed with cyan dots in one main printingscan are also printed with magenta dots. Thus, dot landing states are asshown in FIG. 5B. That is, cyan ink and magenta ink overlap each otherin the printing operation not only for the main dots but also forsatellites. The distribution of satellites is deviated with respect tothe main dots, making the satellites themselves more conspicuous.

In this embodiment, on the other hand, the mask patterns A-D aredistributed as shown in FIG. 8A. In the cyan nozzle columns and magentanozzle columns, and also in their even nozzle columns and odd nozzlecolumns, different mask patterns are applied in the same printing scan.For example, in the first scan of the figure, the cyan even nozzlecolumn uses the mask pattern A, the magenta even nozzle column uses themask pattern B, the magenta odd nozzle column uses the mask pattern C,and the cyan odd nozzle column uses the mask pattern D. In the secondscan, these nozzle columns use different mask patterns than those of thefirst scan. The image data given to the individual nozzle columns areprinted by the four main printing scans successively using the maskpatterns A-D. It is noted, however, that in two nozzle columns ofdifferent colors that print on the same pixels, like cyan even nozzlecolumn and magenta even nozzle column, it is one of the characteristicof this embodiment to use the same mask pattern always in the oppositemain scan directions. For example, the mask pattern A used by the cyaneven nozzle column during the first scan (forward scan) is used in thefourth scan (backward scan) by the magenta even nozzle column.

FIGS. 9A to 9C show dot landing states when a blue, a secondary color,is produced by using the masks of this embodiment. FIG. 9A shows a sumof dots printed in the forward scans, i.e., first scan and third scan.Those pixels printed with cyan dots in the forward scan are not printedwith magenta dots in the forward scan, and similarly those pixelsprinted with magenta dots in the forward scan are not printed with cyandots in the forward scan.

FIG. 9B shows a sum of dots printed in the backward scans, i.e., secondscan and fourth scan. In the backward scans, too, those pixels printedwith cyan dots are not printed with magenta dots. Similarly, thosepixels printed with magenta dots are not printed with cyan dots.

FIG. 9C shows a dot landing state obtained by overlapping the sum offorward scans of FIG. 9A and the sum of backward scans of FIG. 9B. Thecyan dots and the magenta dots that land on the same pixels are printedin the scans of opposite directions. Thus, the two satellites ofdifferent colors land separately on both sides of a main dot. In thiscase, satellites are uniformly distributed with respect to main dots.Further, satellites land in blank areas uniformly, thus reducing gapsbetween dots and granularity caused by gaps and a color difference ofdots. Individual satellites of primary color is less noticeable and lessgranulated than those of secondary color in FIG. 5. Therefore, using dotarrangement of FIG. 9C, a uniform image can be obtained, compared withusing dot arrangement of FIG. 5. Further, the dot arrangement that hassmall satellites located on both sides of the main dots offers anadvantage of an easier image design because the center of gravity ofdots easily stabilizes at the center of each pixel, when compared withthe dot arrangement that has distinctive satellites on only one side ofmain dots.

Although FIGS. 9A to 9C show the effects of this invention in terms ofindividual pixels, FIGS. 17A and 17B show the effects this invention hason images in a wider area. FIG. 17A shows a printed result obtained whencyan dots and magenta dots in the same pixels are printed in the samescan directions by using a conventional mask. FIG. 17B shows a printedresult of this embodiment obtained when cyan dots and magenta dots areprinted in opposite scan directions. An image in FIG. 17B has satellitesdistributed more uniformly with respect to main dots than in FIG. 17A,so it has fewer blank areas and a higher level of uniformity.

In the above, the dot position control method has been explained whichlocates two satellites of different colors on opposite sides of the maindroplet, with cyan and magenta taken as an example. In the printingapparatus of this embodiment, however, black and yellow nozzle columnsare also mounted in addition to the above two colors, and it isimpossible to locate satellites of four colors in all at differentpositions at all times. It is, however, noted that if color combinationsused to produce secondary colors that tend to have higher density andeasily show up visually are properly selected and if the above method isemployed so that the satellites of the selected color combinations arepreferentially arranged in opposite directions, the desirable effects ofthis embodiment can be fully produced. In the above explanation of thedot position control method, it is decided that cyan and magentaconstitutes the above color combination that requires special attention.

Further, while the 4-pass bidirectional printing has been taken forexample in the above explanation, the above desired effects can beobtained as long as the multi-pass printing employs two or more passes.If the mask pattern is configured such that, whatever the number ofpasses, the dots of two colors (cyan and magenta) of interest for thesame pixel are printed in different main scan directions, the satellitescan be made to land uniformly with respect to the main dots andtherefore are evenly dispersed so that they are not easily noticeable,reducing gaps between dots and producing an image of uniform quality. Inthe printing apparatus of this embodiment, a plurality of print modesmay be prepared in advance which, with different number of passes formulti-pass printing, can produce the above effects.

In the above explanation, FIG. 8B has been described to be aconventional, commonly used mask pattern and FIG. 8A a mask pattern ofthis embodiment. In practice, however, the conventional technique doesnot necessarily use the same mask pattern for all colors in the samemain scan. For example, Japanese Patent Application Laid-open No.5-278232 discloses a method in which different ink colors use differentmask patterns in the same main scan. Further, this document alsodescribes as an example a mask pattern used in a 2-pass bidirectionalprinting which prints two dots of different colors of interest on thesame pixel in different main scan directions. Japanese PatentApplication Laid-open No. 5-278232, however, doesn't disclose thearrangement in which satellites of one of two different colors ofinterest and those of the other color are placed on both sides of themain dots. The reason being that the satellites overlapping with themain dots in the forward or backward scanning don't appear both sides ofthe main dots. Because the ejection volume at that time is larger thanthat in current. Accordingly, by the technique of Japanese PatentApplication Laid-open No. 5-278232, a printing operation can not performso that satellites of one color land shifted in the forward directionwith respect to the main dots, while the satellites of the other colorland shifted in the backward direction with respect to the main dots.

Furthermore Japanese Patent Application Laid-open No. 5-278232, however,describes only fixed mask patterns applicable to relatively narrow areasof, for example, 4×4 pixels. The fixed mask pattern is a mask pattern inwhich the print permission pixels are arranged periodically.

FIG. 10 shows example mask patterns for 4×4 pixels, like those describedin Japanese Patent Application Laid-open No. 5-278232. Here, four kindsof mask patterns E-H, complementary to one another, are prepared for a4-pass multi-pass printing. In the figure, pixels painted black or solidpixels represent pixels that are permitted to be printed (printpermission pixel) and blank pixels represent pixels that are notpermitted to be printed (print non-permission pixel). In a real printingscan, the narrow-area mask patterns shown in the figure are repetitivelyarrayed in the main scan direction and sub scan direction for printing.

The embodiment of this invention, on the other hand, applies maskpatterns like those shown in FIG. 26 generally called random masks,rather than the fixed mask patterns like those shown in FIG. 10. In therandom masks, since print permission pixels are randomly arranged, thereis no cyclicity in a relatively wide area. This is a feature of therandom masks. Dot landing states will be explained in the following fora case using fixed masks and for a case using random masks.

FIGS. 11A-11C show how a 4-pass bidirectional printing is performedusing fixed mask patterns of FIG. 10. Here, FIG. 11A represents blueimage data to be printed. Pixels with a blank circle are those where ablue dot is to be formed, i.e., a cyan dot and a magenta dot are to beprinted overlappingly.

FIG. 11B shows dot landing states in each printing scan when the imagedata of FIG. 11A is printed using the mask patterns of FIG. 10. Here,the mask patterns are chosen for each printing scan so that the printingon the same pixel is performed in opposite main scan directions for cyanand magenta.

FIG. 11C show a dot arrangement in an image completed by four mainprinting scans shown in FIG. 11B. Cyan satellites and magenta satellitesare separated and arranged on both sides of the main dots.

FIG. 12 show dot landing states in each printing scan when the imagedata of FIG. 11A is printed using random mask patterns. Here, three4×4-pixel areas are chosen arbitrarily from within a print area and dotlanding states in four printing scans on the area are shown, as in FIG.11B. Unlike the fixed mask patterns, the random mask patterns applied inthis embodiment do not have any regularity such as periodicity.Therefore, the arbitrarily extracted three patterns also have differentdot arrangements.

FIG. 13 shows dot arrangements in an image that is completed by fourmain printing scans in each of the three areas of FIG. 12. As in FIG.11C, cyan satellites and magenta satellites are separated and arrangedon both sides of the main dots but their positions differ among thethree areas.

FIGS. 14A and 14B show images in a wider area (16×16 pixels) that arecompleted by using the fixed mask and the random mask, respectively.Here, satellites that have landed on main dots are not shown. Since theblue main dots are formed by a cyan dot and a magenta dot overlappingone another, if satellites land on these main dots, the color of themain dots is not greatly affected by the satellites. On the other hand,satellites that have landed on blank areas have considerable effects onthe color of the image area of interest. Thus, let us consider thosesatellites that land on white areas.

With the above situations considered, let us refer to FIG. 14A. It isseen that there are far more cyan satellites than magenta satellites.That is, in the case of FIG. 14A, the color of the area of interest(16×16 pixels) is slightly shifted toward cyan from the normal blue.

The mask pattern with a fixed regularity, such as shown in FIG. 11B,easily tunes with regular image data like that of FIG. 11A. Hereby, thedot arrangement of FIG. 11C that is determined by the relation betweenthe image data and the mask pattern appears repetitively in the mainscan direction and the sub scan direction. Therefore, the colordeviation that occurs in a narrow area, such as shown in FIG. 11C, ismaintained in all areas, affecting the entire image. Although we havetake up the pattern of FIG. 11A for example, if a fixed mask pattern isused, the above phenomenon can occur even with other image data.Particularly when a binarization method with some regularity is adopted,as in the case with a dither pattern, the color may shift toward cyan ormagenta and become very unstable depending on the kind of dither patternand its grayscale level.

In contrast to the above, in FIG. 14B showing the dot arrangementobtained through a random mask, the cyan satellites and the magentasatellites are almost equal in number. That is, in the case of FIG. 14B,the color of the area can be said to be almost identical with the normalblue. When a random mask is used, the mask pattern does not tune withimage data whatever image data is entered. Thus, the number of cyansatellites is kept almost equal to that of magenta satellites, with theresult that the color in an even wider area will not shift greatly fromthe normal blue.

For the reasons described above, it is desired that a mask pattern withno cyclicity, such as random masks, be used in order to produce thedesired effect of this embodiment. This is because the use of a fixedmask pattern, such as described in Japanese Patent Application Laid-openNo. 5-278232, results in a color shift due to the tuning between imagedata and mask pattern, reducing an effect for uniformity of multi-passprinting compared with use of a random mask pattern. However, it is ableto obtain an effect of this invention even if using the fixed maskpattern. Therefore this invention doesn't exclude the use of the fixedmask pattern having periodicity.

This embodiment has been described to use different mask patterns A-D ina predetermined order in different printing scans both for cyan ink andfor magenta ink during the 4-pass bidirectional printing. The presentinvention is not limited to this configuration. Where there are aplurality of forward scans and backward scans, the four mask patternsare acceptable even if they don't have the same configuration as long asthe sum of the cyan mask patterns in the forward scans and the sum ofthe magenta mask patterns in the backward scans agree.

As described above, a satellite of a first ink lands shifted in theforward or backward direction with respect to main dots of the first andsecond ink and a satellite of a second ink lands shifted, with respectto the main dots of the first and second ink, in a direction oppositethe direction in which the satellite of the first ink shifts. This makesit possible to produce a uniform image.

Second Embodiment

Now, the second embodiment of this invention will be described. In thisembodiment, too, the printing apparatus explained in FIG. 1 and FIG. 6is applied.

FIG. 15 shows nozzle arrangements in the print head 1102 applied to thisembodiment. This embodiment employs a total of six colors, including alight cyan ink and a light magenta ink with a low dye or pigment densityin addition to the basic four color inks used in the first embodiment.In the figure, denoted 601 is a black ink nozzle column, 602 a cyan inknozzle column, 603 a light cyan ink nozzle column, 604 a magenta inknozzle column, 605 a light magenta ink nozzle column, and 606 a yellowink nozzle column. These nozzle columns of six colors are each comprisedof an even nozzle column and an odd nozzle column, as in the firstembodiment.

FIG. 16 schematically illustrates mask patterns applied to thisembodiment. Shown here are mask patterns for the cyan nozzle column 602and for the light cyan nozzle column 603 in a 4-pass bidirectionalmulti-pass printing. The odd and even nozzle columns, each composed of128 nozzles, have their nozzles divided into eight blocks of 16 nozzlesin the sub scan direction, to each of which one mask pattern is applied.FIG. 16 shows four printing scans, first to fourth scan, and betweeneach printing scan the print medium is fed a distance corresponding totwo blocks. Here, the print head is shown to move relative to the printmedium.

In the FIG. 16, reference symbols A-D represent four different maskpatterns that are exclusive and complementary to one another. That is,image to be printed on one image area on the print medium is completedby successively applying one of the four different mask patterns A-D toeach of the four main printing scans. In this embodiment, too, theindividual mask patterns A-D are random masks with no periodicity.

In the cyan nozzle columns and light cyan nozzle columns and also in theeven nozzle columns and odd nozzle columns, this embodiment appliesdifferent mask patterns in the same printing scans. For example, in thefirst printing scan, the cyan even nozzle column uses a mask pattern A,the light cyan even nozzle column uses a mask pattern B, the cyan oddnozzle column uses a mask pattern C, and the light cyan odd nozzlecolumn uses a mask pattern D. In the second scan, these nozzle columnsuse different mask patterns than those of the first scan. The image datagiven to the individual nozzle columns are completely printed by thefour main printing scans successively using the mask patterns A-D. It isnoted, however, that in two nozzle columns ejecting different ink inconcentration onto the same pixels, like cyan even nozzle column andlight cyan even nozzle column, the same mask pattern is used always inthe opposite main scan directions.

When such mask patterns are employed, pixels that are printed with cyandots in the forward printing scans are not printed with light cyan dotsin the same scan. Similarly, pixels that are printed with light cyandots are not printed with cyan dots in the same scan. Therefore, cyansatellites and light cyan satellites are separated and placed on bothsides of the main dots.

Even with a combination of inks having similar hue (similar color inks),such as cyan dots and light cyan dots, two satellites when they overlapeach other can have greater effects on an image. Therefore, keeping thetwo kinds of satellites as much isolated as possible, as in thisembodiment, is effective in keeping a high level of image quality.Further, as in the first embodiment, the dot arrangement that puts smallsatellites on both sides of the main dots offers an advantage that thecenter of gravity of dots easily stabilizes at the center of each pixel,facilitating an image design, compared with the dot arrangement thatputs distinctive satellites on only one side of the main dots.

In the above, we have described the dot position control method thatputs the satellites of two different colors, e.g., cyan and light cyan,on opposite sides of the main dots. It is possible that the printingapparatus of this embodiment applies the mask patterns that establishthe above relationship also between magenta and light magenta.

In the above two embodiments, explanations have been given to thecombination of cyan and magenta or of cyan and light cyan. The presentinvention of course is applicable to other combinations. For example,the present invention can effectively function in such combinations ascyan and light magenta, and light cyan and light magenta, as long asproblems are caused by satellites of different colors of abovecombination overlapping each other. Further, this invention can also beapplied to a printing apparatus that represents the density of one pixelby using two different ejection amounts of ink droplets which have thesame ink color and the same colorant concentration.

Third Embodiment

Now, the third embodiment of this invention will be described. In thisembodiment, too, the printing apparatus explained in FIG. 1 and FIG. 6is applied.

FIG. 18 shows nozzle arrangements in the print head 1102 applied to thisembodiment. This embodiment replaces a part of the nozzle columns usedin the first embodiment with nozzle columns having different openingdiameters. In the figure, denoted 901 is a black ink nozzle column, 902a cyan ink nozzle column, 903 a magenta ink nozzle column, and 904 ayellow ink nozzle column. Unlike the first embodiment, the even nozzlecolumn and the odd nozzle column are composed of nozzles of differentsizes. For convenience, dots ejected from the odd nozzle column 901 aare defined to be large dots and dots ejected from the even nozzlecolumn 901 b small dots.

FIG. 19 is a schematic diagram showing a mask pattern arrangementapplied in this embodiment. Here are shown mask patterns correspondingto the large cyan nozzle column 901 a and the small cyan nozzle column901 b of the cyan nozzle column 901 used in a 4-pass bidirectionalmulti-pass printing. The odd and even nozzle columns, each composed of128 nozzles, have their nozzles divided into eight blocks of 16 nozzlesin the sub scan direction, to each of which one mask pattern is applied.FIG. 19 shows four printing scans, first to fourth scan, and betweeneach printing scan the print medium is fed a distance corresponding totwo blocks. Here, the print head is shown to move relative to the printmedium.

In the figure, reference symbols A-D represent four different maskpatterns that are exclusive and complementary to one another. That is,image to be printed on one image area on the print medium is completedby successively applying one of the four different mask patterns A-D toeach of the four main printing scans. In this embodiment, too, theindividual mask patterns A-D are random masks with no periodicity.

In the large cyan nozzle column and the small cyan nozzle column, thisembodiment uses different mask patterns in the same printing scan. Forexample, in the first scan of FIG. 19, the large cyan nozzle column usesa mask pattern A, and the small cyan nozzle column uses a mask patternB. In the second scan, these nozzle columns use different mask patternsthan those of the first scan. The image data given to the individualnozzle columns are completely printed by the four main printing scanssuccessively using the mask patterns A-D. It is noted, however, that intwo nozzle columns of large and small nozzles, the same mask pattern isused always in the opposite main scan directions.

If, in an area consists of one pixel in the main scan direction and twopixels in the sub scan direction (one pixel represents a lattice of1200×1200 dpi), a large cyan dot is printed in the first pixel and asmall cyan dot in the second pixel using the same mask for each column,these adjoining pixels are printed in the same scan direction. Toprevent this, the above mask pattern is used in a way that causes thelarge dot and small dot that are supposed to be formed in the adjoiningpixels of the 1×2-pixel area to be printed in different scan directions.

When such a mask pattern as described above is employed, satellites oflarge dot and satellites of small dot are almost uniformly scattered tothe left and right of the main dots, as shown in FIG. 20A, with thelarge cyan dot and small cyan dot as the main dots being arrayed in thesub scan direction in the 1×2-pixel area. As a result, a uniform imagecan be obtained.

FIG. 20B shows a printed state in a wider area as realized by thisembodiment. Satellites that land unevenly to the left and right of themain dots as viewed from the nozzle column direction have considerableadverse effects on the image even if the satellites and main dots are ofthe same color. FIG. 21A shows a dot landing state when the same mask isapplied to the large dot column and the small dot column in the samescan. When a 1×2-pixel area is considered, since the adjoining pixelsare always printed in the same scan direction, satellites land on thesame side of the main dots. FIG. 21B shows a dot landing state in awider area. Compared with FIG. 20B, blank areas and satelliteoverlapping areas show up more distinctly, indicating that the satellitedistribution is uneven. Therefore, keeping the two kinds of satellitesof the main dots that are printed in adjoining pixels in the nozzlecolumn direction (perpendicular to main scan direction; that isconveying direction) as much isolated as possible, as in thisembodiment, is effective in maintaining a high level of image quality.Further, the dot arrangement that puts small satellites on both sides ofthe main dots of the adjoining pixels, as in the first embodiment,offers an advantage that the dot gravity center easily stabilizes at thecenter of the pixels, facilitating the image design, when compared withthe dot arrangement that places distinctive satellites on only one sideof the main dots.

The feature of this embodiment is that, when dots of the same color butof different sizes are printed from two nozzle columns onto two pixelsadjoining in the nozzle column direction (perpendicular to main scandirection), rather than onto one and the same pixel, satellites of twodifferent main dots land on opposite sides of the associated main dots.In other words, a satellite of the first main dot lands on that side ofthe first main dot which is opposite a side of the second main dot wherea satellite of the second main dot lands.

Although in this embodiment an example case has been described wherepixels adjoining in the nozzle column direction are printed with a largedot and a small dot, it is possible to use a combination of dots ofother sizes than the above (for example, medium dot and small dot) or acombination of other colors. For example, a combination of dots of thesame size but of different colors, such as large cyan dot and largemagenta dot or a small cyan dot and small magenta dot, may also be usedin this embodiment and still the intended effects of this invention cansimilarly be produced.

Fourth Embodiment

Now, the fourth embodiment of this invention will be described. In thisembodiment, too, the printing apparatus explained in FIG. 1 and FIG. 6is applied.

In this embodiment, too, the print head explained in FIG. 18 is used asin the third embodiment.

FIG. 22 is a schematic diagram showing a mask pattern arrangementapplied in this embodiment. Here are shown mask patterns to be appliedto a total of four nozzle columns—a large cyan nozzle column and a smallcyan nozzle column of the cyan column 902 and a large magenta nozzlecolumn and a small magenta nozzle column of the magenta column 903—usedin a 4-pass bidirectional multi-pass printing. The odd and even nozzlecolumns, each composed of 128 nozzles, have their nozzles divided intoeight blocks of 16 nozzles in the sub scan direction, to each of whichone mask pattern is applied. FIG. 22 shows four printing scans, first tofourth scan, and between each printing scan the print medium is fed adistance corresponding to two blocks. Here, the print head is shown tomove relative to the print medium.

In FIG. 22, reference symbols A-D represent four different mask patternsthat are exclusive and complementary to one another. That is, image tobe printed on one image area on the print medium is completed bysuccessively applying one of the four different mask patterns A-D toeach of the four main printing scans. In this embodiment, too, theindividual mask patterns A-D are random masks with no periodicity.

In the large cyan nozzle column, small cyan nozzle column, large magentanozzle column and small magenta nozzle column, this embodiment usesdifferent mask patterns in the same printing scan. For example, in thefirst scan of the figure, the large cyan nozzle column uses a maskpattern A, the small cyan nozzle column uses a mask pattern B, the largemagenta nozzle column uses a mask pattern D, and the small magentanozzle column uses a mask pattern C. In the second scan, these nozzlecolumns use different mask patterns than those of the first scan. Theimage data given to the individual nozzle columns are completely printedby the four main printing scans successively using the mask patternsA-D.

It is noted, however, that in a combination of large and small cyannozzle columns, a combination of large and small magenta nozzle columns,a combination of large cyan and magenta nozzle columns, and acombination of small cyan and magenta nozzle columns, the same maskpattern is used always in the opposite main scan directions.

FIG. 23 schematically illustrates the above relationship. Although thisfigure shows the printing scan directions in the mask pattern A, thesimilar relation holds also in the mask pattern B, C and D.

When such a mask pattern is employed, the dot landing state is as shownin FIG. 24A. That is, in a 1×2-pixel area comprising overlapping largecyan and magenta dots and overlapping small cyan and magenta dots,satellites of large dots and satellites of small dots land evenlyscattered to the left and right of the main dots that are arrayed in thesub scan direction. As a result, a uniform image can be produced. FIG.24B shows a printed state of this embodiment when seen in a wider area.

Satellites that land unevenly with respect to the main dots have adverseeffects on the image being formed. FIG. 25A shows a landing state when asecondary color is printed by applying the same mask to the large andsmall cyan nozzle columns and the large and small magenta nozzle columnsduring the same scan. In a 2×2-pixel area, since dots on the same pixelare always printed in the same scan direction, satellites are printed onthe same side of the main dots that are formed in the same pixel. FIG.25B shows printed dots in a wider area. Compared with FIG. 24B, blankportions and satellite overlapping portions show up more distinctly,indicating that the satellite distribution is uneven.

The feature of this embodiment is that, even with a combination ofnozzle columns to print dots of different sizes and a combination ofnozzle columns to print dots of different colors, the positions wheresatellites are printed can be dispersed uniformly with respect to themain dots by properly selecting the order of mask patterns. While thisembodiment has described the dot forming process by taking large andsmall cyan dots and large and small magenta dots for example, thisinvention is not limited to these dots. The similar effects can also beproduced with combinations of nozzle columns of other colors and sizes.

The random mask pattern applied to the above embodiments should bebroadly construed as a “mask pattern without as strong a periodicity asmay be found with fixed mask patterns”. Therefore the random maskpattern is not limited a pattern in which positions of print permissionpixels are decided by randomly.

Furthermore, a mask pattern which can apply to this invention is notlimited to a random mask pattern. For example, a mask pattern having noperiodicity disclosed in Japanese Patent Application Laid-open No.2002-144552 is able to be applied. Furthermore, a mask pattern which hasno periodicity and contains little low-frequency components is appliedacceptably.

This invention functions particularly effectively with a type of ink jetprinting system that has a means to generate a thermal energy changingof state in ink (e.g., electrothermal transducers and laser beams) toeject. With this system, the ink ejection volume can be reduced,realizing an improved print density and resolution. The reduced inkejection volume makes it easier for satellites, the subject of thisinvention, to emerge.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application is a continuation application of PCT application No.PCT/JP2006/313592 under 37 Code of Federal Regulations §1.53 (b) and thesaid PCT application claims the benefit of Japanese Patent ApplicationNo. 2005-200150 filed Jul. 8, 2005 which is hereby incorporated byreference herein in its entirety.

1. An ink jet printing apparatus for printing an image on a print mediumby using a print head which can eject a first ink drop and a second inkdrop, the second ink drop having the same color as and different sizefrom the first ink drop, the ink jet printing apparatus comprising:means for main-scanning the print head relative to the print medium in aforward direction and in a backward direction; and means for controllingejections so that, when the first ink drop and the second ink drop areejected toward the same pixel on the print medium, the first ink dropand the second ink drop are ejected in main scans of differentdirections; wherein a satellite of the first ink drop ejected toward thesame pixel lands shifted in the forward or backward direction withrespect to main dots of the first and second ink drops that land on thesame pixel and a satellite of the second ink drop lands shifted, withrespect to the main dots of the first and second ink drops, in adirection opposite the direction in which the satellite of the first inkdrop shifts.
 2. The ink jet printing apparatus according to claim 1,further comprising: dividing means for dividing a first image data to beprinted by the first ink drop to an area on the print medium that can beprinted by one time of the main scan into M pieces of divided image datafor printing in M times of the main scan corresponding to the area, anddividing a second image data to be printed by the second ink drop to thearea on the print medium that can be printed by one time of the mainscan into M pieces of divided image data for printing in M times of themain scan corresponding to the area; wherein the dividing means dividesthe first and second image data so that the ejections of the first inkand the second ink toward the same pixel can be executed in the mainscans of different directions.
 3. The ink jet printing apparatusaccording to claim 2, further comprising: memory for storing a firstmask patterns for dividing the first image data into M pieces and asecond mask pattern for dividing the second image data into M pieces,wherein the dividing means divides the first image data into M piecesbased on the first mask pattern and divides the second image data into Mpieces based on the second mask pattern.
 4. The ink jet printingapparatus according to claim 3, wherein the mask patterns have noperiodicity.
 5. The ink jet printing apparatus according to claim 3,wherein the first and second mask patterns are each patterns in whichprint permission pixels are arranged randomly.
 6. An ink jet printingmethod for printing an image on a print medium by using a print headwhich can eject a first ink drop and a second ink drop, the second inkdrop having the same color as and different size from the first inkdrop, the ink jet printing method comprising the steps of: main-scanningthe print head relative to the print medium in a forward direction andin a backward direction; and controlling ejections so that, when thefirst ink drop and the second ink drop are ejected toward the same pixelon the print medium, the first ink drop and the second ink drop areejected in main scans of different directions; wherein a satellite ofthe first ink drop ejected toward the same pixel lands shifted in theforward or backward direction with respect to main dots of the first andsecond ink drops that land on the same pixel and a satellite of thesecond ink drop lands shifted, with respect to the main dots of thefirst and second ink drops, in a direction opposite the direction inwhich the satellite of the first ink shifts.